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Li metal is very attractive as a negative electrode material for high-energy-density all-solid-state batteries owing to its high specific capacity and low electrochemical potential. However, short-circuiting of batteries upon formation of Li dendrites is a serious issue that hinders its successful practical application. An advantage of all-solid-state batteries is their inherent safety at high temperatures. In this study, the high-temperature performance of all-solid-state Li-metal batteries containing sulfide-glass electrolytes was investigated. Symmetric cells with Li3PS4 electrolytes exhibited better Li dissolution-deposition performance at 100degC than at 25degC. In addition, inserting Au thin films at the Li/Li3PS4 interface enabled stable operation of the symmetric cells at high current density (1.3 mA cm[?]2) and large areal capacity (6.5 mAh cm[?]2) without short-circuiting. All-solid-state Li-metal batteries with Au thin films (Li/Au/Li3PS4/LiNi1/3Mn1/3Co1/3O2) exhibited high rate performance at 2.4 mA cm[?]2 and long lives of over 200 cycles at 100degC. The dissolution of the Au thin films into the Li metal is a possible reason for the enhanced electrochemical performance. These results indicate that interface modification and optimizing operating temperature are promising strategies to achieve all-solid-state batteries with high energy densities.

Electronic and ionic conductivities of positive composite electrodes composed of LiNi1/3Mn1/3Co1/3O2 and Li3PS4 were measured by both DC and AC techniques. Two cell configurations of (a) electron-blocking cells and (b) ion-blocking cells were applied for the measurements. The conductivities determined by the DC technique showed a good agreement with those measured by the AC technique, which suggests that both the DC and AC techniques are useful for the measurements of the electronic and ionic conductivities. The electronic conductivities for the composite electrodes at the state of charge (SOC) 0% were lower than the ionic conductivities. The electronic conductivities drastically increased at SOC 50% and became higher than or equal to the ionic conductivities. Cell capacities were evaluated based on the electronic and ionic conductivities. At a high current density of 1.3 mA cm-2, cell capacities seemed to be associated with the ionic conductivities of the composite electrodes.

Li2S-FeSx (x = 1, 2) composite positive electrode material, consisted of low-crystalline Li2S, was prepared by the combination process of the thermal heating and the mechanical milling. The electrochemical tests demonstrated that the Li2S-rich composite sample (Li2S : FeS = 4 : 1 mol) cells showed relatively high initial charge capacity (ca. 600 mAh*g[?]1). The discharge capacity was ca. 330 mAh*g[?]1 with the normal electrochemical cycling, and it was enlarged to ca. 730 mAh*g[?]1 after the stepwise pre-cycling treatment. Ex situ XRD measurements showed the improved reversibility of the Li2S peaks for Li extraction/insertion reactions after the stepwise pre-cycling treatment. Also, ex situ S and Fe K-edge XAFS measurements indicated the improved reversibility of the valence states and local structures around S and Fe atoms after the stepwise pre-cycling treatment. Thus, the stepwise pre-cycling treatment was effective for improving the structural reversibility of the Li2S-FeSx composites, particularly with Li2S-rich composition, for Li insertion/extraction reactions, resulting in the improved electrochemical performances of the cells.

Amorphous oxide thin films in the pseudobinary system Li4SiO4-Li3PO4 and Li4GeO4-Li3PO4 were fabricated using pulsed laser deposition (PLD) with a KrF excimer laser. The ionic conductivities of obtained thin films were 10[?]8 to 10[?]6 S cm[?]1 at room temperature. Bulk-type all-solid-state batteries using LiCoO2 particles surface-modified with ortho-oxosalt thin films were constructed to assess the influence of oxide coatings on cell performances. Higher ion-conductive oxide thin films studied here are useful for decreasing the interfacial resistance of all-solid-state cells. The interfacial resistance was lowest in the cell using Li3.5Si0.5P0.5O4 coatings as a buffer layer with estimated thickness of ca. 45 nm. We investigated the dependence of interfacial resistance on film thickness for the oxide systems fabricated in this study. Among them, much lower interfacial resistance was achieved in the cells using Li3PO4 coatings with film thickness of less than 45 nm. All-solid-state cells using Li3PO4-coated LiCoO2 charged and discharged without marked capacity-fading during 30 cycles. Larger discharge capacity after cycling tests was achieved in a cell with a smaller interfacial resistance. The cell using Li3PO4-coated LiCoO2 operated at a high current density of 6.4 mA cm[?]2 with a capacity of 54 mAh g[?]1.

Effects of modification of the electrode/electrolyte interface with thin films on impedance profiles and rate capabilities of all-solid-state glass ceramic/ cells were investigated. Large resistance was observed at the -electrode/sulfide-electrolyte interface in all-solid-state cells. The interfacial resistance was decreased by oxide coatings on ; the effective coatings were a lithium-ion conductive glassy film and an insulative film. The coating film decreased the interfacial resistance more effectively when the coating amounts increased from (film thickness was ca. ). However, of coating exhibited higher interfacial resistance than noncoating because the thick coating film acted as a high-resistance layer. Temperature dependence of the interfacial resistance suggests that the resistance decrease was achieved mainly by an increase of pre-exponential factor rather than by a decrease of activation energy for ion conduction at the interface. At room temperature, all-solid-state cells with -coated were discharged even under the high current density, .

Lithium dissolution/deposition behavior of Al-doped Li7La3Zr2O12(LLZ) was investigated in terms of grain size of LLZ-sintered bodies. One LLZ had smaller grain sizes of primarily less than 1 μm (LLZ_SG), and the other one had larger grain sizes of 5–20 μm (LLZ_LG). The total resistance of a Li symmetric cell using LLZ_SG was smaller than that using LLZ_LG at 100°C. The cell using LLZ_SG was stably cycled at 100°C without short-circuiting at a high current density of 1.3 mA cm−2, while the cell using LLZ_LG was stably cycled at current densities below 0.26 mA cm−2. Stronger bonding at grain boundaries for LLZ_SG was expected to contribute to the improvement of cyclability.

Effects of modification of the electrode/electrolyte interface with Li2O–SiO2thin films on impedance profiles and rate capabilities of all-solid-state In∕80Li2S⋅20P2S5glass ceramic/LiCoO2cells were investigated. Large resistance was observed at the LiCoO2–electrode/sulfide–electrolyte interface in all-solid-state cells. The interfacial resistance was decreased by 0.06wt%oxide coatings on LiCoO2; the effective coatings were a lithium-ion conductive Li2SiO3glassy film and an insulative SiO2film. The Li2SiO3coating film decreased the interfacial resistance more effectively when the coating amounts increased from 0.06to0.6wt%(film thickness was ca. 10nm). However, 0.6wt%of SiO2coating exhibited higher interfacial resistance than noncoating because the thick SiO2coating film acted as a high-resistance layer. Temperature dependence of the interfacial resistance suggests that the resistance decrease was achieved mainly by an increase of pre-exponential factor rather than by a decrease of activation energy for ion conduction at the interface. At room temperature, all-solid-state cells with Li2SiO3-coated LiCoO2were discharged even under the high current density, 6.4mAcm−2.